The present invention relates to a process for treating a gas by adsorption, of the type in which use is made of a pressure swing adsorption treatment unit, commonly referred to as a PSA unit, and in which said unit is regulated via pressure equalizations and/or the elution gas.
PSA units are commonly used for the separation and/or purification of feed gas, especially in the fields of producing hydrogen, helium or carbon dioxide, of drying, of separating the constituents of air, etc.
“PSA-H2” units which produce substantially pure hydrogen are used with feed gases of varied origin, formed for example from gases resulting from steam reforming, refinery gases or coke-oven gases, or else formed from waste gases from ethylene or styrene production units, or from cryogenic hydrogen-carbon monoxide separation units.
Generally, a PSA unit consists of several adsorbers which follow, with a time lag, an operating cycle, subsequently referred to for convenience as a “PSA cycle”, which is uniformly distributed over as many phase times as there are adsorbers in operation, and which is formed from basic steps, namely the steps of:                adsorption at substantially a high pressure of the cycle;        co-current depressurization, generally from the high pressure of the cycle;        counter-current depressurization, generally to the low pressure of the cycle;        elution at substantially the low pressure of the cycle; and        repressurization, from the low pressure of the cycle to the high pressure of the cycle.        
Co-current depressurization generally comprises one or more equalization steps and at least one purge-providing step which supplies the elution gas.
Repressurization generally comprises the corresponding equalization steps and a final repressurization with production gas or elution gas.
These steps define the characteristic pressures of the PSA.
FIG. 1 corresponds to a cycle with three complete equalizations, the pressures at the end of equalization for the depressurization adsorber being respectively P1, P2, P3 and P′1, P′2, P′3 for the repressurization adsorber with equality between these pressures (P1=P′1, etc.).
The elution gas is extracted, for its part, between P3 and P4.
FIG. 2 corresponds to a cycle comprising incomplete equalizations characterized by a residual pressure difference at the end of the steps between the depressurizing adsorber and the repressurizing adsorber: DP1, DP2, DP3.
FIG. 3 corresponds to a cycle where the purge-providing step is simultaneous with the equalization 3. The gas resulting from the depressurization between P2 and P3 is distributed between elution and equalization. In case of a simultaneous step, the ratio of gas allocated to the elution is an important parameter.
The staging of the pressures P1, P2, P3, P4, etc. between the high pressure and the low pressure of the cycle is one of the important characteristics of the cycle which set the performance thereof.
Indeed, they determine the amounts of gas exchanged between adsorbers and the amount of elution gas. In this way, they act both on the extraction efficiency of the PSA and on the productivity of the adsorbent, and thereby on the investment.
Hereafter, the description relates to the operation of a PSA unit in the steady state, i.e. outside of transient periods during which the unit is started up or shut down, which generally correspond to special cycles set up for this purpose.
The main operating constraint of a PSA unit in the steady state consists of the degree of purity of the product. Under this operating condition, the treatment performance level of a PSA unit are then generally optimized either in order to maximize the extraction efficiency (amount of gas produced/amount of this gas present in the feed gas), or in order to minimize the energy consumed or more generally in order to reduce the operating costs, or in order to minimize the investment.
Obtained in this way is a nominal operating cycle of the PSA unit, which is determined directly as a function of the nominal operating conditions (flow rate of the feed gas, flow rate of the treated gas, composition of the feed gas, operating temperature of the unit, pressures, etc.).
When the operating conditions depart from the nominal conditions, it has been proposed to date to regulate the operation of the PSA unit by adjusting one or more parameters of the nominal cycle so as to guarantee that the treatment performance levels remain above predefined minimum limits. Two regulations that come under this approach are:                “capacity” regulation, which consists in modifying the duration of the phase time of the cycle as a function of the variation in the feed gas flow rate; and        “purity control” regulation, which consists in modifying the phase time as a function of the purity of the treated gas.        
It is advisable here to define what is understood by cycle time and phase time (or more simply phase).
As has been described above, an adsorber will therefore begin an adsorption period until it is loaded with the constituent(s) to be stopped at the high pressure, then will be regenerated by depressurization and extraction of the adsorbed compounds before being restored in order to restart a new adsorption period. The adsorber has then carried out a “pressure cycle” and the very principle of the PSA process is to link these cycles together one after the other; it is therefore a cyclic process. The time that an adsorber takes to return to its initial state is referred to as the cycle time. In principle, each adsorber follows the same cycle with a time lag that is referred to as the phase time or more simply the phase. The following relationship therefore exists:
Phase time=cycle time/number of adsorbers and it is seen that the number of phases is equal to the number of adsorbers.
There may be any number N of adsorbers, but generally N is between 2 and 32, more typically between 4 and 16.
In practice, there are a large number of options for performing the two-fold (capacity, purity) regulation, the result of which is to operate the PSA simultaneously under good purity and efficiency conditions. Generally, the variations in flow rate modify the first-order cycle time and the analysis comes as correction. The actions of the regulators are to regulate in order to ensure that the purity will be maintained. The actions may be asymmetric, i.e. rapid and large if the pollution increases and on the contrary slow and measured in the case of too high a quality of the hydrogen. As a reminder, various practices exist in order to anticipate the variations in purity of the hydrogen such as carrying out an analysis in the adsorbent bed, for example at 95% of the total volume, and not at the outlet or analyzing the gas during one of the co-current (equalization or purge-providing) steps.
This basic regulation which ensures the performance levels is generally supplemented by additional regulations. Indeed, the modification of the cycle time in order to adapt the adsorption time to the new conditions (flow rate, composition of the feed, etc.) results in all the phases of the cycle being modified in the same way. Depending on the cycles, it is possible to maintain the steps at their initial duration and supplement up to the duration of the new phase, assumed here to be lengthened, with a dead time (with or without closure of the valves depending on the configuration of the circuits) or by lengthening some of the steps. In order to make the flow rates as regular as possible, if the cycle allows it, the counter-current blowdown, purge-providing and repressurization steps will thus be lengthened.
Ultimately, under stabilized operation, assumed here to be at reduced feed flow rate, a production having the required purity and the nominal efficiency of the cycle used is obtained.
Nevertheless, the fact remains that the unit operating under these new conditions is overdesigned with respect to a design which would have been made specifically regarding this case. The amortization of the initial investment comes down to a lower hydrogen production and the specific cost is therefore higher. Over long operating periods of the unit, this operation is economically poor.